Reverse Osmosis Membrane Maintenance Guide: Flush Before Startup & Clean Before Shutdown—This Is the Correct Operation!

However, in actual working conditions, many operators tend to overlook two critical operations—startup flushing and shutdown cleaning—which ultimately lead to membrane fouling, performance degradation, and even direct scrapping of the membrane elements. This article will break down the necessity and standardized procedures of these two operations to help extend the service life of your reverse osmosis membranes.

1. No Flushing Before Startup = Membrane Elements "Chronic Poisoning"

After an RO system shuts down, concentrated liquid, trapped impurities, and microbial slime layers remain inside the membrane elements. Direct startup without flushing will trigger a series of hazards caused by these "residues":

• High-concentration salts disrupt the osmotic pressure balance on the membrane surface, causing membrane sheet swelling and deformation.
• Residual impurities instantly clog membrane pores, leading to a sharp rise in system differential pressure.
• Microbial slime accelerates biofouling on the membrane surface, shortening the chemical cleaning cycle.

Standardized Startup Flushing Procedure

1. Pretreatment-linked flushingStart the raw water pump, multimedia filter, and cartridge filter first to allow qualified pretreated water to flow through the RO system without activating the high-pressure pump. Open the RO concentrate and permeate drain valves, and flush for 5–10 minutes to discharge residual concentrated liquid from the membrane housing.
2. Low-pressure flushing for air removalStart the high-pressure pump and adjust the frequency converter to a low-pressure state (pressure controlled at 0.2–0.4 MPa, complying with industry standard safety specifications to avoid membrane sheet damage from excessive pressure). Continue flushing for 3–5 minutes to expel air inside the membrane elements, preventing membrane rupture caused by air compression under high pressure.
3. Start permeate production after parameter qualificationMonitor inlet water turbidity, SDI value, and residual chlorine content during flushing to ensure all indicators meet the membrane element inlet requirements. Simultaneously observe concentrate flow, permeate flow, and system differential pressure. Once parameters stabilize, gradually increase the pressure to the design value (approximately 0.7 MPa for brackish water conditions, up to 6.9 MPa for seawater conditions), close the permeate drain valve, and switch to normal production mode.

1. No Cleaning Before Shutdown = Membrane Elements "Direct Scrapping"

If membrane elements remain in a "loaded" state for an extended period after system shutdown, the damage will be far more severe than that caused by skipping startup flushing. During shutdown, residual salts and organic matter on the membrane surface will crystallize, forming stubborn inorganic scales; microorganisms will multiply rapidly under suitable temperatures, forming biofilms attached to the membrane surface. These contaminants directly damage the membrane's desalination layer, resulting in irreversible performance degradation.

Shutdown Cleaning Operations by Scenario

1. Short-term shutdown (within 24 hours): Simple flushing for membrane preservationApplicable to temporary shutdowns, equipment maintenance, etc. The core objective is to displace concentrated liquid inside the membrane and inhibit microbial growth.
• After turning off the high-pressure pump, keep the pretreatment system running, and flush the membrane system with qualified pretreated water (RO permeate is preferred to avoid secondary pollution from impurities in pretreated water) at low pressure for 10–15 minutes, completely replacing the concentrated liquid inside the membrane with clean water.
• Close all valves after flushing, and keep the membrane housing filled with clean water to prevent membrane sheet drying and shrinkage.
2. Medium-term shutdown (1–7 days): Chemical flushing with bacteriostatic agentApplicable to factory maintenance shutdowns, holiday shutdowns, etc. Bacteriostatic measures are required to prevent microbial contamination.
• Complete the short-term shutdown flushing procedure first, then prepare a 1.0% sodium bisulfite solution (an industry-standard concentration with stable bacteriostatic effect) or a dedicated membrane bacteriostatic agent. Use a low-pressure circulation pump to inject the solution into the membrane system, ensuring the membrane elements are fully immersed in the bacteriostatic solution.
• Close all valves to maintain system sealing. Regularly check the pH value of the bacteriostatic solution during storage (maintaining pH 3–4 optimizes bacteriostatic effect) and replenish chemicals promptly if fluctuations occur.
3. Long-term shutdown (over 7 days): Chemical cleaning + preservation and storageApplicable to seasonal shutdowns, long-term equipment idleness, etc. Thorough contaminant removal and proper preservation are required.
• First, perform routine chemical cleaning: For inorganic scales (calcium carbonate, calcium sulfate), circulate a citric acid solution (pH adjusted to 2–3); for organic and biological contaminants, use a sodium hydroxide solution combined with a dedicated non-oxidizing bactericide (sodium hypochlorite and other chlorine-containing oxidants are strictly prohibited, as their strong oxidizing properties irreversibly damage the polyamide membrane desalination layer). After cleaning, rinse the system with RO permeate until the effluent pH reaches neutrality.
• Prepare a dedicated membrane preservation solution (formulated according to membrane manufacturer requirements; commonly 1% sodium bisulfite or 1% formaldehyde solution containing bacteriostatic and corrosion-inhibiting agents). Inject the solution into the membrane system via low-pressure circulation to ensure full immersion of membrane elements. Seal the inlet and outlet of the membrane housing, and regularly inspect the preservation solution to prevent leakage and deterioration.

1. Key Precautions for Flushing/Cleaning

1. Water quality requirementsRO permeate is the preferred water source for flushing/cleaning. If pretreated water is used, it must meet the following criteria: turbidity < 1 NTU, SDI < 5, residual chlorine < 0.1 mg/L to avoid secondary membrane contamination.
2. Temperature controlFlushing water temperature must be maintained between 5–45℃. Excessively high temperatures accelerate membrane material aging, while low temperatures reduce the reaction efficiency of cleaning chemicals.
3. Pressure and flow controlAdopt low-pressure flushing throughout the process; overpressure operation is strictly prohibited to prevent membrane sheet deformation under pressure. Ensure sufficient flushing flow to guarantee that every flow channel of the membrane element is fully flushed.
4. Chemical selectionOnly use chemicals specifically designed for RO membranes. Strong acids, strong alkalis, chlorine-containing oxidants (e.g., sodium hypochlorite), and other substances that damage membrane materials are strictly forbidden. Sodium hypochlorite is only suitable for non-polyamide membranes in special scenarios, with strict control over concentration and contact time.

Conclusion

For reverse osmosis membranes, "30% depends on selection, 70% on maintenance". Although startup flushing and shutdown cleaning seem like trivial operations, they are critical to ensuring membrane element longevity. Only by strictly implementing standardized flushing and cleaning procedures can premature aging and scrapping of membrane elements be avoided, enabling long-term stable operation of the RO system and reducing operation and maintenance costs.

If your factory encounters issues such as inaccurate parameter control or incomplete contaminant removal during RO membrane flushing and cleaning, please leave a comment below. We provide 24/7 targeted solutions! Feel free to contact us at +86 15936809132—we can customize solutions based on your actual working conditions to help optimize your filtration system configuration.